The present invention relates to cochlear implant systems, and more particularly to a type of stimulation applied to a patient through a cochlear implant that enhances the effectiveness of the cochlear implant.
The currently most widely used sound and speech processing strategies in cochlear implants are CIS and SPEAK. Both arc sequential pulsatile strategies, whereby each and every current pulse delivered to the auditory nerve represents a processed quantum of acoustic information that is derived from sound that has been recorded by the microphone of the cochlear implant system. Each stimulation pulse is modulated in amplitude and/or pulse width by the amount of energy and/or information that is recorded at the output of a corresponding filter and/or envelope extraction stage of the speech processor, frequently referred to as "channel". In the CIS strategy, if the output of a processing channel is zero for a given time period, the corresponding stimulation pulse will be of such amplitude as to fall below the patient's audible threshold, i.e. the pulse does not cause an auditory perception; nonetheless, it is delivered with the purpose of carrying a parcel of acoustic information (which happens to be zero at some points in time) to the auditory system.
In recent years, the modeling of acoustic nerve properties, as well as current laboratory research, indicate that better speech understanding and sound clarity for cochlear implant users may possibly be achieved by applying pulses to the auditory system at very high rates. The rationale is that very high pulse rates elicit a desirable stochastic response pattern in the auditory neural system. The purpose for generating very high stimulation rates is thus independent of the delivery of acoustic information to the auditory neural system.
Very high sequential pulse rates result in increasingly narrower pulse widths. Such increasingly narrower pulse widths make it difficult for the patient to perceive changes in loudness. This is because loudness as perceived by the patient is largely a function of charge (i.e. the combination of pulse width and pulse amplitude). In order to accomplish sufficient loudness growth with very narrow pulses, the pulse amplitude has to increase in proportion to the decrease of pulse width. Disadvantageously, very high pulse rates and ensuing narrow pulse widths are not practical for information delivery to the auditory neural system of transcutaneous cochlear implant users because of loudness growth limitations that result from limited compliance voltage. (The compliance voltage is the maximum voltage available within the cochlear implant device from which stimulation pulses may be derived.) Thus, it is seen that improvements are needed in the way high sequential pulse rates are delivered to a patient in order for such stimulation to be effective, e.g., in order to allow the patient to be able to perceive loudness.